An example of car engine starter-cum-electric power generators of the related art is disclosed in PTL 1. A three-phase winding (stator) of a rotary electric machine is connected in a triangular shape, and a three-phase bridge circuit that is configured to have power semiconductor switches (power MOSFETs) is connected to the three-phase winding. Each of the power semiconductor switches is controlled to be ON and OFF at predetermined timing by a control circuit, converts DC power of a battery to AC power so that a three-phase AC current flows in the stator when operated as an electric motor, such as during car engine start, and is driven to rotate by the engine and rectifies a three-phase AC current induced in the stator for conversion into a DC current and supply to the battery when operated as an electric power generator.
In this example, a drive circuit that controls a field winding (rotor) and a current amount thereof is also provided, and an output torque as the electric motor and an electric power generation amount as the electric power generator can be variable when a field winding current amount is adjusted. In PTL 1, an avalanche effect of each of the power semiconductor switches is used as means for suppressing an excess voltage during load dump such as battery terminal disconnection in an electric power generation operation, and a rated voltage thereof is set as a value required for excess voltage suppression. This is because device size and costs increase when a Zener diode is used between DC terminals of the electric power conversion unit.